A 56-year-old woman was referred to our hospital because persistence of fever and inflammatory syndrome after antibiotic treatment for pneumonia. In the past, no special feature. Physical examination revealed only--at first--a pneumonia. Two weeks after, associated with the cough, she developed an acute renal failure. The laboratory revealed an inflammatory syndrome associated with proteinuria, hematuria and anti-neutrophil cytoplasmic antibodies. The CT of thorax shows aspecific infiltrations. The renal biopsy, in the context of the patient, diagnosed an microscopic polyangiitis. All the symptom resolved with the initiation of corticoid and cyclophosphamide treatment.